The present disclosure relates to a developing device used in an image forming apparatus utilizing an electrophotographic process, such as a copier, a printer, a facsimile machine, or a multifunction peripheral thereof, and to an image forming apparatus including the developing device. In particular, the present disclosure relates to a developing device that replenishes two-component developer containing toner and carrier and discharges excessive developer, and to an image forming apparatus including the developing device.
In the image forming apparatus, a latent image is formed on an image carrier such as a photoreceptor, and the developing device develops and visualizes the latent image into a toner image. One of such developing devices adopts a two-component developing method using two-component developer. In this type of developing device, two-component developer (hereinafter also referred to simply as developer) containing carrier and toner is stored in a developing container, a developing roller is disposed for supplying developer to the image carrier, and a stirring member is disposed for conveying and stirring the developer in the developing container so as to supply the developer to the developing roller.
In this developing device, the toner is consumed by the developing operation while the carrier is not consumed and remains in the developing device. Therefore, the carrier, which is stirred with the toner in the developing container, is deteriorated along with an increase in stirring frequency, and as a result, electrification performance of the carrier to the toner is gradually deteriorated.
Therefore, there is known a developing device that replenishes developer containing carrier into the developing container and discharges excessive developer, so as to prevent deterioration of electrification performance.
For example, as for a system for replenishing carrier and toner into a developing container, there is known a developing device in which a stirring member includes a first feeding portion for feeding developer in a developing container, a second feeding portion disposed on the downstream side in the feeding direction of the first feeding portion and constituted of an opposite spiral blade for feeding the developer in the opposite direction to the first feeding portion, and a third feeding portion disposed on the upstream side in the feeding direction of the second feeding portion so as to feed the developer in the same direction as the first feeding portion and to discharge the developer from a developer discharge outlet.
With this developing device including the first feeding portion, the second feeding portion, and the third feeding portion, when the developer is replenished into the developing container, the developer is stirred and fed to the downstream side in a feeding chamber by rotation of the first feeding portion. When the opposite spiral blade of the second feeding portion rotates in the same direction as the first feeding portion, a feeding force in the opposite direction to the feeding direction of the first feeding portion is applied to the developer. This feeding force in the opposite direction stops the developer and makes it bulky, and hence excessive developer overflows the second feeding portion, moves to the developer discharge outlet, and is externally discharged.
In addition, in the developing device including the first feeding portion, the second feeding portion, and the third feeding portion, the developer discharge outlet has an inner diameter smaller than the feeding chamber, and a rotation shaft of the stirring member is formed to have a diameter decreasing toward the developer discharge outlet at a boundary part between the feeding chamber and the developer discharge outlet.